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The Long-Lived Legacy of the Cambrian's "Wonderful Life"

A model of Peytoia (formerly called Laggania). This bizarre Burgess Shale creature was a close relative of Anomalocaris. Model created by Espen Horn, image from Wikipedia.

Anomalocaris was an unlucky casualty of history. One of the wonderfully strange creatures of the 505 million year old Burgess Shale, this segmented horror - with flexible, spiky arms and a shutter-like mouth - was one of many evolutionary oddities thought to have been wiped out by an extraordinarily unfortunate turn in the history of life. For the past several decades, paleontologists have thought that Anomalocaris and other Cambrian curiosities disappeared by the end of the period. They were either winnowed away before the Cambrian curtain fell, or were wiped out in a last catastrophic pulse. With the weirdos gone, surviving lineages proliferated into the Ordovician seas and formed the foundation for life as we know it.

In Wonderful Life, Stephen Jay Gould forcibly argued that the decimation of spectacular invertebrates at the end of the Cambrian was a matter of luck.* Anomalocaris* wasn’t inherently inferior, or the survivors objectively superior. The quirks of life simply stacked the deck against some forms of ancient life while giving a pass to others for intricate ecological reasons we haven’t been able to discern.

Whatever the reason they disappeared, though, the extinction of some Cambrian forms had a dramatic effect on the history of life on earth. The species that ultimately lost – such as Anomalocaris – embodied other, ultimately unrealized evolutionary possibilities. Imagine what the seas would look like if such bizarre animals survived to the present? In an example that perfectly melds my love of prehistory and monster movies, Gould speculated that if Anomalocaris and kin had been given a reprieve from extinction, they could have been the inspiration for fantastic creature features. “Why not a Steven Spielberg film with a crusty seaman sucked into the cylindrical mouth of a sea monster, and slowly crushed to death by multiple layers of teeth lining a circular mouth and extending well down into the gullet?”, Gould asked. This was somewhat antithetical to Gould’s big point that if the Cambrian extinction had been cancelled, life on earth would have turned out so differently that our species probably wouldn’t have evolved, but I still like the colorful example.

Gould wrote Wonderful Life as an argument against our conceited belief that evolution always picks the “best” creatures to survive, and that, by extension, our species is a product of a long pattern of predictable progress. I still think his point is spot-on and is essential to our understanding of how evolution unfolds. But as paleontologists have kept picking away at the fossil record, Gould’s view of unfortunate Cambrian geeks who suffered a terminal case of bad luck has transformed into something even more perplexing. Many of those Cambrian oddballs actually did survive, and, curiously, there may not have been a Cambrian mass extinction, after all.

Researchers have only started to uncover the new story over the past few years. In 2009, Gabriele Kühl and co-authors described Schinderhannes bartelsi from the Devonian slate of Germany. This creature was a cousin of Anomalocaris – one of the “great appendage” invertebrates with highly-modified, spiky arms under their heads – but one that lived about 100 million years after such creatures were thought to have gone extinct. And, the following year, Kühl and Jes Rust redescribed another Cambrian exemplar found in the same deposits. This second creature is called Mimetaster hexagonalis, and is quite similar to a Burgess Shale arthropod called Marrella – a multi-legged invertebrate with an ornate head shield decorated with long, backward-pointing spikes. Furthermore, paleontologists Derek Briggs, Peter Van Roy, and others have documented a whole array of Cambrian-type creatures found in the 488-472 million year old rock of Morocco. These fossils fall between the time of the Burgess Shale and the specimens found in the Devonian of Germany, and show that many of the creatures that so fascinated Gould and other researchers actually persisted for millions of years after the close of the Cambrian.

Now paleontologists have identified another Cambrian holdout. The fossil comes from the roughly 300 million year old deposits of Mazon Creek, Illinois, and, as described by Joachim Haug and collaborators, the animal seems to be closely related to a long-lost lineage of worm-bodied, many-limbed invertebrates called lobopodians.

From a wider perspective, lobopodians are still with us. Velvet worms that crawl through the undergrowth of tropical forests - carried by their short, conical legs - are descendants of one Cambrian group. But there was another lineage of lobopodians that disappeared long ago. These lobopodians had long, tubular legs, and were thought to have disappeared at the end of the Cambrian. The new find extends the range of these stilt-legged lobopodians by about 200 million years.

The researchers named the creature Carbotubulus waloszeki. At first glance, the fossil doesn’t look like much more than a yellow smear on red rock. As labeled in the paper, though, the creature’s large head, a set of enigmatic appendages jutting from that head, and nine pairs of walking limbs are immediately apparent. The whole fossil is small – only about 35 millimeters long – but Carbotubulus is preserved in enough detail to see that this was a long-lived member of a much more ancient lineage.

And the strange nature of Carbotubulus stands out when compared to another fossil found at the same locality. A lobopodian of the modern, stubby-legged sort – called Ilyodes inopinata – has also been found at Mazon Creek. Whereas Ilyodes looks much like a velvet worm, Haug and colleges point out, Carbotubulus more closely resembles Cambrian forms that were previously thought to have disappeared many millions of years before. Another Cambrian freak survived far longer than anyone expected.

I have to wonder if there are other, as-yet-unrecognized Cambrian survivors out there. Given the gaps between the Cambrian fossils and the late survivors in Germany and Illinois, there must be. In fact, Mazon Creek is famous for another invertebrate that may be a candidate for such a distinction.

In 1966, Eugene Richardson formally described the “Tully Monster” – Tullimonstrum gregarium to specialists – from a collection of puzzling fossils extracted from Mazon Creek. These animals – which apparently lived in great numbers among the estuaries of ancient Illinois – had a cylindrical bodies, eyes perched on long stalks, a pair of vertical fins at the back half of the body, and a long proboscis tipped with tooth-studded pincher. No one knows exactly what this thing is. The Tully Monster is one of the longest-running mysteries in paleontology. Yet, at a superficial level, the Tully Monster recalls more ancient Cambrian enigmas such as the controversial Nectocaris, the soft-bodied Vetustovermis, or, possibly, the schnozzle-faced aberration Opabinia. This is just armchair speculation on my part, but I have to wonder if part of the mystery of the Tully Monster is that it’s yet another Cambrian survivor, divorced from its closest relatives by 200 million years. Even if not, there are undoubtedly other bewildering fossils of Cambrian survivors in museum collections and locked in the rock record. We are really only just beginning to understand the legacy of what Gould justly dubbed the Cambrian's "wonderful life."

[Hat-tip to paleontologist Kenneth De Baets, who told me about this paper and also blogged it himself.]